The popularity of the Atkins Diet and the South Beach Diet -- both of which limit carbohydrates -- plus the proliferation of low-carb processed foods can be seen as evidence that carbs have become the new bogeyman of the American diet. But what is the science behind this low-carb revolution? And do these diets work? Here are the views of Dr. Arthur Agatston, author of The South Beach Diet; journalist Gary Taubes; nutritionists Marion Nestle, Jeanne Goldberg and Aviva Must; and Dr. Dean Ornish, author of Eat More, Weigh Less.

Cardiologist, author The South Beach Diet

[W]hat happened at the time this [obesity] epidemic started were the national recommendations on low-fat, [high]-carb diets, and the response of the food industry, with all the great low-fat, high-carb, processed carbohydrate foods. Basically the idea was that the low-fat, high-carb diet was to mimic underdeveloped countries, which are low fat, high carb, no heart attacks, no strokes, no obesity. What the American food industry produced, though, was carbohydrates where the fiber as well as the nutrients were taken out. …

And in fact, what ended up was an unintended experiment in this country. There've been countries, societies that have been high-fat, who have done well. Some Mediterranean countries, the island of Crete was studied very, very carefully; the Greenland Eskimos have been studied. Those are high-fat societies that do fine as far as cardiovascular disease and obesity. And we mentioned the underdeveloped world, [which] tends to be low-fat, high-carbohydrate, also does well.

No society was ever low-fat, high-processed carbohydrate. That was a new experiment, as well as throwing in trans fats, those of the hydrogenated oils found in commercial baked goods and cookies that are over-the-counter. Those turn out to be the worst fats, and the worst for both our blood vessels and long-term weight loss.

What the processed carbohydrates do is, they are rapidly digested. That causes swings in our blood sugar where we're hungry soon after we finish a meal, and we're ready to eat some more. Basically Americans are walking around hungry all the time, and [I believe strongly that's the primary cause] of the epidemic of obesity.

Journalist, author "What If It's All Been A Big Fat Lie?" (New York Times Magazine, July 7, 2002)

Everyone agrees that insulin is the hormone that controls the deposition of sugar and carbohydrates and fat in your body. They agree that if insulin levels are high, you'll preferentially store calories as fat; and that as long as insulin levels stay high, you won't be able to get to that fat to use it for fuel. They agree that carbohydrates will raise insulin levels more than -- fat doesn't have an effect on insulin, although if you force-feed enough calories, you can [raise] it. All of that is given.

What they don't agree is that somehow the carbohydrates, the actual macronutrient content of the diet, will do this. [Scientists] will say a calorie is a calorie is a calorie. They'll admit that a calorie of carbohydrates has an entirely different effect on your hormonal system than a calorie of fats. They'll admit that your hormones can control your weight; that insulin and estrogen have effects on weight, hunger, and body weight regulation. But they will never go from the step where they say: Hey, maybe the amount of carbohydrates and the kind of carbohydrates in the diet will have an effect -- through their effect on insulin, through insulin's effect on the deposition of calories, through that effect on hunger -- [on] being a functional diet.

To me, it's almost mystifying, because I've interviewed people who have done research on each step of the way. And then I say, "This should be the null hypotheses. You should assume carbohydrates cause weight gain until proven otherwise, not the other way around. Why don't you believe that?" And they'll say, "Well, because my rats get fat on fat." And it's true. Rats will get fat on dietary fat, fatter than they will on carbohydrates. So then the question becomes: Are rats a good animal model for human obesity?

So this theory would explain a big part of the rise of obesity on the high-carb diets?

This theory would explain a big part of the rise of obesity on the high-carbohydrate diet. Yes. Basically it says: The things that are making us fat -- sugars, high-fructose corn syrup, these kinds of easily digestible sweets, flour -- some people may get fat even if they eat vegetables. I really don't know because they've never done the research. Because they've been so convinced that somehow the type of calorie is irrelevant, I'm not sure this is still an open question. But the theory would explain why we started putting on weight just when we started thinking "if we would just reduce the amount of fat in the diet, that'll make the difference."

It's not just the calories. That's the point. It's the effect of the calories on the hormones, and the effect of the hormones on how your body decides to use the calories you're eating -- is it going to burn them as fuel or store them as fat -- and that effect on hunger.

But clearly the quantity of calories has to be going up. You're saying the mechanism for that is that the carbs make us hungrier?

The quantity of calories appears to be going up. Whether we would get fat anyway -- we've all had this experience where our weight fluctuates, apparently independent of how much we seem to be eating. If you chronically change your insulin levels, you will change your weight by doing it. So the question is: Can you do that by increasing calories, or do you do that simply by increasing carbohydrates and keeping the calories constant? Good research in the '80s by established researchers showed you can do that just by changing the amount of carbohydrates. ...

What would be your prescription for curbing the obesity epidemic?

There's compelling evidence that low-carbohydrate diets work. Clinical trials suggest they work, [as does] anecdotal evidence for 150 years. People, when they wanted to lose weight, gave up sugar, gave up starch and we all grew up believing this. Even into the '70s. Jane Brody, the New York Times health reporter, writes a bestselling book in 1985 where she tells everyone to go out and eat starches to their hearts' content, but first she says we all grew up believing that what causes obesity is bread, rice, pasta, potatoes, sugar.

It's fascinating. For 150 years since Jean Anthelme Brillat-Savarin in 1825 writes The Physiology of Taste and says, "I know what cause obesity. Just talk to fat people. They eat too much starches and sugars." He says this. "I have 500 conversations over the year with stout people, and each one, they're telling me, 'I love the potatoes. I love the rice. I love the bread.'" Since then, it's sort of been institutionalized that carbohydrates -- starches and sugars -- make you fat.

The '70s come around. We decide that fat gives you heart disease. We have to eat low-fat diets, low-fat diets heavy in starches and sugars, and we start getting fatter. ...

You're not talking about a diet with no carbs, just a reduced amount?

The underlying philosophy is this kind of Paleolithic diet theory. It's what we ate during the 2 million years that we were hunter-gatherers on this planet. The fact that we were hunter-gatherers for 2 million years suggests it was an extraordinarily successful evolutionary adaptation. The question is: What did we eat during these 2 million [years] when we left the jungle, the trees, went down into savanna and started surviving on whatever we could hunt or gather? That's the philosophy. The answer is, probably considerable meat, very low glycemic index, hard-to-digest roots and starches, and fruits and berries that look nothing at all like the beautiful Fuji applies you can buy at your local market now. Some carbohydrates, but whatever it was, it wasn't refined. It wasn't sugar. It wasn't flour. It wasn't easy to digest. That's my going theory. If this theory's right, the diet we evolved to eat is probably the correct diet.

[A]n alternative hypothesis: Fat is no longer the bad guy. The bad guy is carbohydrates, driving towards a new theory of glycemic index.

Right. I don't see these as new theories. I see these as part of a discussion about what you do when you're overeating calories, because it's very clear that taking in more calories than your body can handle easily deranges metabolism in all kinds of ways. That's where the "good carb, bad carb," "good fat, bad fat" issues become very important, because a very high carbohydrate diet on top of a diet that already has too many calories is clearly very difficult for the body to handle.

The food industry is now gearing up to produce low-carb products.

Yeah. We're going to now see a huge proliferation of low-carbohydrate products. To the extent that these products help people reduce their calorie intake, I think they'll be fine. Once people find out that if they're still overeating calories, these products aren't going to work, then it's just another fad. But all of it is a distraction from what I see as the most important issue, which is to eat more fruits and vegetables and watch your calories.

Professor, Tufts University Friedman School of Nutrition

My prediction is that the low-carbohydrate foods will, long run, be just about as successful as all the low-fat foods were. People are demanding them. Industry is responding to the demand. At the end of the day, it's not how people really want to eat. And really, food is fun, it's pleasure, it's what people like to do. These aberrant diets don't fit with what people really have become accustomed to, what they enjoy. It's not going to be the answer to the weight problem.

Are you saying all diets are artificial so they don't [work]?

No. What I'm saying is that the diet which sustains is probably a diet which is relatively higher in carbohydrate, complex carbohydrate, on the higher end in protein, and relatively low in fat.

But diet is only part of the story. You've got to put the exercise piece into the equation. Until recently, that's really been left out. It isn't that you can't burn off whatever you eat unless you're a marathoner and train all the time. But you do have to be physically active to manage to control your weight, unless you are willing to eat a very restricted diet. …

Some people feel fats have got too bad a deal and carbs have got too good a deal -- [for example] the New York Times article. What is a nutritionist's response?

When we swing to the low-carbohydrate, low-fat, I think, "Oh no, not again." I really wish we could get a consistent message of balance intake and expenditure. Eat lots of whole grains, fruits and vegetables, limited amounts of fat.

I think for the public, really understanding all of this chemistry about trans fats and saturated fats, it runs counter to the way people choose a diet. If you go up to somebody in the supermarket and say, "Excuse me. Can you tell me whether you looked at the trans fat information? Or what information did you look at, on the food label?" The odds overwhelmingly are that they didn't look at the nutrition label. Many of the labels, they've looked at and learned about and moved on. But if they looked at anything, they looked at calories, and hopefully they looked at calories and serving sizes.

I think we really need to get back to helping people to understand what portions are. If we could get back to the portion issue, and understanding that the plate of spaghetti on the bottom of the food guide pyramid was never meant to be one serving of spaghetti. Many people looked at that and said, "Oh my God, if I ate six of those, I'd be like a house in a week." That was never the intent. The intent was to make a recognizable graphic that looked like pasta.

I think we're trying to throw too much at people, and we [should] really try to get back to a whole message about what should your diet look like over a week, and yes, it's okay to have some of your favorite foods some of the time, just not every day, all of the time, and as much as you like. …

The idea that you can take a starchy food like a potato and it's turned into [sugar] very quickly, is genuinely novel, shocking to people.

… The problem with glycemic index is that it's extremely complicated. Setting aside whether controlling the spikes in blood sugar are a good idea or not, the form in which the food is cooked affects the glycemic index of the food. The mix of foods in which it's eaten affects the glycemic index. Measuring the glycemic index of carrots doesn't do any good unless you know what other foods the carrots are going to be eaten with.

So in my view, using glycemic index as something that we teach people how to live by is like selling them a pet rock. To me, it's unfathomable. It doesn't translate. Even if there is, down the road, proven to be some effect on controlling blood sugar more evenly throughout the day, I think we've got to be a little bit more practical about how we do it. And the notion of telling people to limit potato consumption -- I mean, yes, limit french fry consumption; yes, [don't] eat giant portions; but edging [potatoes] into a bad food category, to me, just doesn't make a whole lot of sense as a practical thing.

Associate Professor, Tufts University School of Medicine

The low-carb diet that is so fashionable now, I believe, was not very different than the drinking man's diet that I remember my father being on in the '60s. There have always been diets. There was the grapefruit diet, and there was the ice cream diet. There are always diets that focus on something, and many of them work. They work because of the boredom factor, or because they're so highly restrictive that it's just really hard to maintain caloric intake in the face of them, and some of them may, at least in the short term, make it easier to be more restrained in your eating.

I don't think that that's so much new, as perhaps the increasing sophistication of marketing of these different diets. I think that people who understand how to sell have gotten more and more sophisticated, and that may be contributing to some of the consumer confusion and information overload in terms of what they should or shouldn't be eating.

Why are the Atkins diet and diets like it so incredibly popular? And also, why do they seem to work?

They're popular because they tell people what they want to hear, and they work to some degree because they're based on a half-truth. I debated Dr. Atkins many times before he died, and so I'm very familiar with the Atkins diet. The half-truth is that Americans eat too many calories. One reason is that they eat too much fat, because fat is so dense in calories. The other is that they eat too many simple carbohydrates, and that's the area that we both agreed on.

The problem with simple carbohydrates -- and these are things like sugar, white flour, white rice, alcohol, which your body converts to sugar -- is that you get a double whammy. You get all these calories that don't fill you up, because when you go from, say, whole wheat flour, which is complex, to white flour, which is a simple carbohydrate, you've removed the fiber and the bran. Those ordinarily would fill you up before you get too many calories. You can only eat so many apples. You're going to get full before you get too many calories. But when you remove the fiber and the bran, you can consume virtually unlimited amounts of, say, sugar or white rice without getting full.

If you're trying to lose weight, it's better to avoid or reduce the intake of simple carbohydrates. We agree on that.

The other reason that simple carbohydrates cause people to gain weight, besides the fact that you can eat so many without getting full, is that they get absorbed quickly. So your blood sugar zooms up, your pancreas makes insulin to lower your blood sugar, which is good, but insulin also causes you to convert calories into fat, which is not good. We both, Dr. Atkins and I, agreed that Americans eat way too many simple carbs, and so if you're eating a typical American diet, which is high in simple carbs, and you go on an Atkins-type diet, you may lose weight because you're eating fewer simple carbs.

But you could lose even more weight by eating fewer simple carbs and less fat, because both are the reasons why people get too many calories, and rather than harming your health, you'd be enhancing it by eating the way that I suggest. Because it's not only what you exclude in your diet; it's what you include that's protective. A number of studies have shown that there are substances that have anti-cancer, anti-heart disease, anti-aging properties: things like phytochemicals and bioflavonoids, carotenoids, retinals, isoflavones. There's a whole alphabet soup of these. Where do you find these protective substances? With few exceptions, you find them in fruits, vegetables, whole grains, and soy products and other legumes. To tell people they shouldn't be eating these foods saddens me.